The endocannabinoid system in mammals plays a role in many physiological functions and is an important therapeutic target. In addition to the two main cannabinoid receptors there is increasing evidence that there are other, as yet unidentified, CB receptors in humans. Cannabinoid receptors have been identified in many different species, including some invertebrates, but they have not been identified in the nematode Caenorhabditis elegans. In C. elegans a conserved insulin signaling pathway acts during development to regulate the decision to proceed with reproductive growth or form the dauer larva, an alternate larval stage that allows the worm to survive periods of population overcrowding and starvation. In a chemical screen, I discovered that the synthetic cannabinoid receptor antagonist AM251 suppressed dauer formation in the insulin receptor mutant daf-2. The ability of AM251 to prevent dauer formation is inhibited by the presence of synthetic cannabinoid receptor agonists, suggesting the presence of a nematode cannabinoid receptor. This idea is further strengthened by the identification of components of a worm endocannabinoid system including multiple endocannabinoid ligands and biosynthetic enzymes that modify endocannabinoid levels. In this respect, I have discovered that the worm contains several endocannabinoids that are structurally related to the CB receptor ligands in mammals. Levels of these endocannabinoids are elevated following inhibition of the worm ortholog of fatty acid amide hydrolase, a hydrolytic enzyme involved in endocannabinoid degradation and inactivation. Furthermore, some of the endocannabinoids are elevated during starvation while others are decreased, suggesting a role in feeding and response to starvation. Finally one of the worm endocannabinoids, eicosapentaenoyl ethanolamide, an isoform of the mammalian endocannabinoid arachidonoyl ethanolamide, acts in the same way as AM251 to promote reproductive growth in daf-2 mutants. Since the mammalian type 1 and type 2 cannabinoid receptors do not appear to have orthologs in worms, I propose that C. elegans expresses a receptor that acts as a functional homolog of mammalian cannabinoid receptors. The overall goal of this project therefore is to identify the molecular target of cannabinoids in worms and to characterize in more detail the nematode endocannabinoid system. The identification of a new cannabinoid receptor subtype in nematodes will bring the strengths of C. elegans as a major model system to the study of endocannabinoid physiology and increase our understanding of how cannabinoid and insulin signaling pathways interact. Finally the discovery of a worm cannabinoid receptor may lead to the identification of a novel human CB receptor, which has the potential to provide a new therapeutic target for cannabinoid based drugs. PUBLIC HEALTH RELEVANCE: The discovery of an endocannabinoid system in the nematode roundworm Caenorhabditis elegans and the absence of orthologs of the two main mammalian receptors suggest the existence of a novel cannabinoid receptor. The identification of this receptor in worms may help in the identification of new mammalian cannabinoid receptors and provide new targets for the development of cannabinoid based therapeutics.